The dark fringe of data

The search for gravitational waves is an epic endeavour ....…

The dark fringe of data

Astronomy has often been compared to looking for a needle in a haystack, but Dr Chris Messenger makes it sound even harder than that. He has spent a significant part of his career in the quest to detect gravitational waves from “continuously emitting sources” such as neutron stars or pulsars, but 20 years since he first set out on his search at the University of Birmingham, studying theoretical physics, he still hasn't managed to find them.

Neutron stars are very dense collapsed stars which can rotate at approximately 1,000 times per second. Because they are so dense and exert such a strong gravitational pull from the core, they also have very smooth surfaces, but gravitational waves are only emitted by oscillating or rotating asymmetrical objects – for example, if there is something sticking up from the surface. The tiny imperfections on a neutron star, however, are much more like pimples than mountains – they are only one or two millimetres in height on an object which is roughly the size of a city such as Glasgow, approximately 10km wide with a mass twice as great as the Sun.

This degree of difficulty may explain why Messenger and other leading scientists have not yet detected continuous gravitational waves, despite employing the “most stringent detection methods” ever developed to search “wide area parameter space.” Researchers can simulate the sound of the continuous gravitational waves, and would therefore be able to recognise them if they detected their signature waveform, but so far, there have been no concrete results.

Messenger may have to wait to detect continuous gravitational waves (please see sidebar), but he has played a key role in the first two detections of compact binary gravitational waves (please see sidebar) by LIGO (the Laser Interferometer Gravitational-wave Observatory) in September and December 2015. These two historic events were both caused by collisions between two black holes, which released a huge burst of energy lasting just a split second, rather than the weak harmonic signal you would get from a continuously emitting source such as a pulsar. The team in the LIGO Scientific Collaboration expected that the first thing they'd detect would be a coalescence of two neutron stars, simply because astronomical observations prove their existence, unlike the lack of evidence for merging black holes, but that just goes to prove that probability can be full of surprises.

The search for gravitational waves is an epic endeavour. The sources could be anywhere out there, but according to Messenger, “you would need all the computing power in the world to do an optimal search,” and that is why he and his colleagues in the data analysis groups have focused on novel mathematical methods to improve the sensitivity of the detector – for example, using innovative machine learning techniques (a type of artificial intelligence that enables computers to learn without being programmed to do so).

After gaining his PhD in astrophysics at Birmingham, Messenger then worked at the University of Glasgow, and at the Albert Einstein Institute for Gravitational Research in Hanover, Germany, before spending three years at the University of Cardiff and returning to Glasgow in 2013, to take up a position as a Lord Kelvin Adam Smith Research Fellow. After ten years working as a post-doc in research labs, Messenger was delighted when he was appointed a Lecturer in Gravitational-Wave Astrophysics at the University of Glasgow early this year, but says that most researchers lead precarious lives, without the security of long-term positions.

When he first went back to Cardiff in 2010, Messenger thought he would continue his research into continuous gravitational waves, focusing on candidate stars in the Scorpius constellation, but he shifted his attention to “compact binary coalescence” – looking at black holes and/or neutron stars in compact orbit around each other which are likely to merge because of the energy lost into gravitational waves.

Whilst at Cardiff, and subsequently at Glasgow, Messenger continued to develop new data analysis methods for studying the properties of neutron stars and trying to use them to as cosmological probes. Together with a colleague from the University of Mississippi, he discovered that it was possible to measure how fast these objects were receding from us as the Universe expands. This made the future detection of binary neutron star mergers even more valuable to scientists, since it would enable them to measure this expansion using gravitational waves.

According to Messenger, the mathematics used to study gravitational waves – including Bayesian inference methods* – are already highly advanced, and the next challenge is to develop the next generation of detectors to generate more detailed data. “We can identify and understand the data,” he explains, “and we know the real thing when it comes. The data contains spurious detector noise as well as astrophysical data. We spend a lot of time studying the data to try to disentangle them.”

Messenger says he's been “lucky” to chair the group tasked with “determining the statistical significance of potential detections” – the probability of candidate events. The data analysts can spend a lot of time dealing with “detector glitches” which may interfere with the signals.

“We get lots of glitches in the noise spectrum,” says Messenger, “and our job is to understand their properties.” Glitches come along which look like signals from Space, and it is possible that both detectors, thousands of miles apart, may detect similar glitches at the same time, but that is statistically very unlikely. To detect false alarms, the researchers use a method called a “time slide” to analyse unusual events dating back over a few days or weeks, and this enables them to see if the event was a coincidence or something more significant.

The checking process involves introducing a series of artificial time shifts to create a much longer data set in which they search for signals as strong as (or stronger than) the candidate event. The time shifts are more than ten milliseconds (the time it takes for light to travel between the detectors) to ensure the artificial data sets don't contain any real signals, but only coincidences in noise, to see how often a coincidence mimicking the wave would appear. This analysis provides the false alarm rate – how often they expect to measure such a seemingly loud event that was really just a noise fluctuation.

This process doesn’t happen every day, but false alarms must be taken seriously, just in case. Says Messenger: “We generate hundreds of thousands of triggers that can’t possibly be gravitational waves.”

The data analysts have learned from past experience to be very cautious to make any premature claims, but Messenger also believes that even though they may have been “bad science,” the false alarms of the 1960s at least sparked interest in the search for gravitational waves, and alerted future researchers to the need for bullet-proof verification. “You could go and find all sorts of signals out there,” he explains. “That is why we look at populations of sources, and have set up four data analysis groups – focusing on compact binary coalescence, bursts, continuous waves and the stochastic background.

According to Messenger, the current detector will not even reach its “design sensitivity” level for another two years and, meanwhile the researchers are designing much more sensitive detectors which will greatly expand their horizons. Once they have recorded more detections, says Messenger, they will have much more meaningful data on entire populations of black holes –what he calls “ensembles of detection” – and machine learning promises further advances, shedding light on what Messenger calls the “dark fringe” of space. What the team at LIGO has already discovered will lead to the creation of a new field of science, so who knows what they’ll find on the dark fringe of data in future?

Gravitational waves

Continuous gravitational waves are produced by a single spinning massive object such as an extremely dense star or a neutron star. Any imperfections in the spherical shape of this star will generate gravitational waves as the star spins. The gravitational- wave signal it emits also remains constant, but at a frequency equal to twice the frequency at which the star is spinning.

Compact binary inspiral gravitational waves are produced by orbiting pairs of massive and dense (hence “compact”) objects such as white dwarf stars, black holes and neutron stars. Each binary pair creates a characteristic series of gravitational waves, but the waves they generate are all called “inspiral,” as they revolve around each other. As they orbit, they send off gravitational waves and, as they lose this energy, they inch together faster and faster until they are locked in a fatal embrace. In the frequency range of the LIGO detectors, the signals we pick up from compact binary inspiral gravitational waves are short in duration (several seconds to less than a second long) and increase in frequency as the stars orbit faster. The first detection of gravitational waves captured the final fraction of a second of the merger of two black holes 1.3 billion light years away.

Neutron stars

A neutron star is the collapsed core of a very large star. With a typical radius of about 10km and a mass of about two times the Sun, they are the smallest and densest stars known to exist. They result from an explosion, combined with gravitational collapse, that compresses the core. If the core is too dense, it continues collapsing to form a black hole. Neutron stars are very hot and so dense that a cubic inch of neutron-star material would have a mass of approximately 13 million tonnes. They also have strong magnetic and gravitational fields. As the core collapses, its rotation rate increases up to several hundred times per second. Some neutron stars emit beams of electromagnetic radiation that make them detectable as pulsars. There are an estimated 100 million neutron stars in the Milky Way.

Biography

Dr Chris Messenger was, until recently, a Lord Kelvin Adam Smith Research Fellow in the Institute of Gravitational Research (IGR), and was appointed a Lecturer in Gravitational-Wave Astrophysics in early 2017. He is a leading authority in *Bayesian inference methods, which allow us to probe and constrain the properties of individual sources of gravitational waves and also the properties of their underlying population – i.e., understand the properties of that population from the few sources we have detected by taking into account the sources that we haven’t detected. For several years, Messenger chaired the working group charged with developing methods for estimating the rate of gravitational-wave events to be expected, and determining the significance of the source candidates.

* Bayes’ theorem is used to “update the probability for a hypothesis as more evidence or information becomes available”